Bovine staphylococcal mastitis is a frequent problem for the dairy industry, and leads to estimated annual economic losses of $184 per cow per year. This corresponds to a U.S. total of $1.7 billion per year for milk producers and milk processors. These losses arise from reduced milk yield, reduced compositional quality, lower product quality, and increased veterinary costs.
Mastitis is transmitted from cow to cow at milking time. Staphylococcus aureus (S. aureus) is a major pathogen that infects both humans and animals which accounts for 15 to 30% of intramammary infection cows. Staphylococcus infections are characterized by their persistence and their deleterious effects on milk production and quality. Current therapies and preventative treatments for staphylococcal mastitis rely heavily on sterilization techniques, selective culling of animals with chronic recurring mastitis, and the use of β-lactam antibiotics such as cepharin and penicillin derivatives (Bramley and Dodd, J. Dairy Res., Craven and Anderson, J. Dairy Res., 51:513-523, 1984). Also, numerous attempts have been made to develop vaccines, but none have stood the test of time (Derbyshire and Smith, Res. Vet. Sci. 109:559, 1969; Nelson L. et al., Flem. Vet. J., 62 Suppl., 1:111; Rainard et al., Flem. Vet. J. 62 Suppl., 1:141; Watson et al., Proc. Int. Symp. Bovine Mastitis Indianapolis, 73).
Although sterilization techniques and the use of antibiotics have had a positive impact on dairy animal health and milk production, the prognosis for the elimination of S. aureus infection is poor, with often less than a 15% cure rate. This problem may be attributable to incomplete penetration of the antibiotics and/or sequestration of the bacteria within the host cells, leading to relapse of the infection once treatment has ended (Craven and Anderson, supra). The widespread use of antibiotics in dairy animals is also of great concern to the consumer. One problem is accidental exposure of the consumer to the antibiotic drug that can induce a strong immune response resulting in anaphylaxis. There is also a concern that the overuse of antibiotics selects for microorganisms that are resistant to the antibiotic. Many S. aureus strains have already acquired resistance to commonly used antibiotics such as ampicillin and penicillin. Such prevalent problems have made it necessary to discard milk for a period of up to 96 hours after antibiotic treatment of an animal, resulting in an enormous waste of milk product and cost to milk producers.
There is a need for the development of an improved approach to treating mastitis infections.